Apparatus for inspecting a.c. electric relay systems during the operation thereof by use of a high frequency modulated signal

ABSTRACT

In an apparatus for inspecting an A.C. responsive relay system during operation thereof a pulse train is provided having distinctly higher repeating frequencies than those input signals to the relay indicative of a real fault and which pulse train is modulated by A.C. voltages having frequencies and wave forms similar to those input signals for the relay indicative of a real fault. The pulse train so provided is created by mixing a pair of first and second pulse trains modulated by A.C. voltages in a pair of ring-modulators. The pulse A.C. voltages used for the modulation have distinctly higher frequencies than those of the real fault input signals for the relay. The A.C. voltages for the modulation feed the positive half waves thereof to a first multivibrator associated with one of the ring-modulators to thereby provide the first train. The A.C. voltages for the modulation, further, feed its negative half waves to a second multivibrator associated with the other ring-modulator to establish the second pulse train. Both of the multivibrators include means for regulating the respective time constants thereof whereby the pulses within the first and second pulse trains are distributed in the combined pulse train with extremely small time intervals between the resultant adjacent pulses, respectively. Thus, the combined pulse train so formed and used as the inspection signals may be easily distinguished from input signals indicative of a real fault which are applied to the relay system.

United States Patent Muraoka [54] APPARATUS FOR INSPECTING A.C.

ELECTRIC RELAY SYSTEMS DURING THE OPERATION THEREOF BY USE OF A HIGHFREQUENCY MODULATED SIGNAL [72] Inventor: Masahide Muraoka, Tokyo, Japan[73] Assignee: Tokyo Shibaura Electric Co. Ltd.,

Kawasaki-shi, Japan [22] Filed: Feb. 5, 1970 [21] Appl. No.: 9,009

[30] Foreign Application Priority Data Feb. 10,1969 Japan ..44/948$ [52]US. Cl. ..324/28 R, 317/27 R [51] Int. Cl ..G0lr 31/02 [58] Field ofSearch ..324/28 R, 28 CB; 317/36 D,

RING-MODULATOR [451 June 6, 1972 Primary Examiner-Rudolph V. RolinecAssistant Examiner-R. 0. Corcoran Attorney-Bums, Lobato & ZelnickABSTRACT In an apparatus for inspecting an A.C. responsive relay systemduring operation thereof a pulse train is provided having distinctlyhigher repeating frequencies than those input signals to the relayindicative of a real fault and which pulse train is modulated by A.C.voltages having frequencies and wave forms similar to those inputsignals for the relay indicative of a real fault. The pulse train soprovided is created by mixing a pair of first and second pulse trainsmodulated by A.C. voltages in a pair of ring-modulators. The pulse A.C.voltages used for the modulation have distinctly higher frequencies thanthose of the real fault input signals for the relay. The A.C. voltagesfor the modulation feed the positive half waves thereof to a firstmultivibrator associated with one of the ring-modulators to therebyprovide the first train. The A.C. voltages for the modulation, further,feed its negative half waves to a second multivibrator associated withthe other ring-modulator to establish'the second pulsetrain. Both of themultivibrators include means for regulating the respective timeconstants thereof whereby the pulses within the first and second pulsetrains are distributed in the combined pulse train with extremely smalltime intervals between the resultant adjacent pulses, respectively.Thus, the combined pulse train so formed and used as the inspectionsignals may be easily distinguished from input signals indicative of areal fault which are applied to the relay system.

7 Claims, 7 Drawing Figures PATENTEnJuu s 1912 SHEET 3 OF 4 TIME FIG. 38

FIG. 3C

TIME

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60 w 63 18 5| 56 pz 53 58 54 t @[QIQE 49 Q 50 4515 E E EMS I 47 I @1RING -MOD ULATOR OSCILLATOR 39 El"? E: 7o RELAY" RING-MODULATORAPPARATUS FOR INSPECTING A.C. ELECTRIC RELAY SYSTEMS DURING THEOPERATION THEREOF BY USE OF A HIGH FREQUENCY MODULATED SIGNAL BACKGROUNDOF THE INVENTION A The present invention generally relates to apparatusfor inspecting relay systems, and more particularly to apparatus forchecking the protecting relay systems of electric power equipments, forexample, electric power transmission lines, A.C. electric machines, suchas A.C. power generators, synchronous motors, power transformers havinga large electric capacity, and the like.

As is well known in the art, it is necessary to occasionally inspect theprotective relay systems used in the electrical engineering fields. Thusfor example, it is desirable to inspect the protecting relay systemused; in electric power transmission lines to protect the same from anyfault which may occur therein during its operation. This periodicinspection or checking will insure a successful operation of therelaying systems, without any misoperation thereof, at the instant oftime in which any fault has occurred during the normal or real operationof the power transmission lines.

In such protective relay systems, such as, transmission lines there areusually employed one or any suitable combination of distance relays,directional distance relays, zero-phase over voltage relays andinstantaneous fault detectable relays, such as undervoltage relays, overcurrent relays and power flow relays, as well as transmitters andreceivers for carrier protec-' tion. In order to prevent any damage tothe electric power equipment, it is imperative that the respectiverelays in the protective systems operate without any misoperationthereof. However, often a fault may occur within the relays themselvesfor-a long period of time. Such faults may be a breaking of a relaycoil, a faulty transistor, a defect in an integrated circuit and thelike.

Thus, if the protective relay system is misoperating it is impossible toremove faults which may occur in the transmission lines being protected,and as a result a failure will cause the power transmission system toassume a state of abnormal operation.

One particular inspection device for relay systems in which a usual highfrequency A.C. voltage signal, distinctly diflerent from the localcommercial frequency, has been employed as the inspecting signal. Wherethe high frequency signal is applied to the input of the relay systems,the latter may respond to the signals in such a fashion that the relaysystem will function to simulate the case in which an actual faultexists, when in fact no real fault present in the relay systems. In thiscase, because the frequency of the inspecting signals is distinct fromthe commercial frequency, no interference will result therebetween and,in addition, it is possible to inspect the relay systems without havingto cut off the other input signals, that is, the commercial frequencyinput signals to the relay systems.

ln the aforementioned inspecting device, there has been employed aconstant high frequency source having a relatively constant amplitude.Such a source is suitable for making a relatively Simple inspection forsome relay elements which are for example simply checked by the on-offstates of their contacts. However, there are various complicatedinspections which are required for relay elements, e.g.- comparison ofthe phase difference between and among two or more A.C. quantities.Also, as is well known, it is sometimes required to check the functionsof distance relays and directional distance relays in which the formeris responsive to the distance from the installed point thereof to afailed point on the transmission lines and the latter responds to thedirection of the fault presented on the lines as well as to the distanceto the failed point as prescribed above. As is also well known, in thedistance relay or the directional distance relay, the variations of thephase differences between the line voltages and the line currents of thetransmission line are measured by the prescribed relay systems which arelocated near the terminal of a section of the transmission line to beprotected. Any suitable output signals of the relay systems, which maybe interrupted by the failed section of the transmission line, will beemitted from the relay output in the form of either mechanical on-offsignals for a mechanical relay or electrical on-off output signals for astatic relay where the amplitudes of the prescribed phase differenceexceeds a predetermined value.

While somewhat satisfactory, one problem in the past is that it has beenshown that the inspecting systems using usual high frequency signals asthe inspecting signals have often resulted in insufficient results whenattempting to make examinations analogous to real cases. Thus, forexample, the usual high frequency signals cannot be employed for thepurpose to detect a minor phase difference in the electrical anglesbetween and among two or more A.C. electric quantities which isnecessary for the distance relay systems.

In particular there are various ways in which switching transistors wereemployed to turn-on and turn-off within the protecting relays fortransmission lines and the like. Such onoff transistors would beemployed in the distance relay systems and may receive the inspectionsignals, each of which would periodically vary through a zero and amaximum level in accordance with the wave forms thereof. Thus, theon-ofitransistors would tum-on and turn-ofi, and then the relay systemswould misoperate without taking into consideration any comparison of thephase difference between the different electric quantities.

SUMMARY OF THE INVENTION It is therefore an object of the presentinvention to provide an improved apparatus for inspecting relay systemsby using a pulse train which has relatively high repeating frequencieswhich are clearly distinct from the usual relay input signals and whichpulse train is modulated by A.C. voltages having frequencies similar tothose of the usual relay input signals.

In accordance with the invention it is an object to provide aninspection system for a relay in which any switching transistor thereofis not subjected to a misoperation resulting from the fact that thepulse waves in the pulse train pass through zero points every repeatingcycles.

Briefly stated in accordance with a preferred embodiment of theinvention there is provided at least one electrical relay system havingat least two inputs to which are applied separate A.C. input signalshaving substantially sine wave forms respectively and at least oneoutput which is capable of producing there from the desired results fromthe two input signals. There are also provided a pair of rectangularwave generators which alternatively generate repeatable pulses at theoutputs thereof respectively. A pair of ringmodulators are provided inorder to separately modulate both of the pulse trains from therectangular wave generators with an alternating current at a commercialfrequency which is similar to the frequency employed in the electricpower systems to be protected by means of the present relay systems. TheA.C. modulated pulse trains are then combined with each other to provideinspection signals analogous to the usual inputs to the relay systems.An extremely short time interval is present between the adjacent pulsesin the resultant pulse train. There is also provided a low pass filterat the input of the relay to be inspected, which filter functions topass through the usual commercial frequency signals to be conducted tothe relay, but not to pass through the high frequency inspectionssignals which may be directed to the commercial frequency source. Theinput of the relay system also includes means to make the off statesbetween adjacent pulses in the inspection signals relatively smooth.

BRIEF DESCRIPTION OF THE DRAWINGS This invention can be more fullyunderstood from the following detailed description when taken inconnection with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of an inspection system for a relayembodying the present invention;

ry winding 37 with a center tap 38.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG.1, there is generally shown by example a directional distance relay witha reference numeral 10, and it can be assumed that the relay includestransistorized or integrated circuits having at least one switchingtransistor therein. The relay has a pair of inputs 11 and 12 and anoutput 13 one of the inputs, namely input 1 1, is used for receiving thesystem voltages and the other input namely input 12 is employed forreceiving the system currents. In order to simplify the illustration,only a section of an electric power transmission line 14 to be protectedby the directional distance relay is shown. The input 11 of the relay 10is connected to an end terminal 15 of the section 14 through a potentialtransformer l6 and an auxiliary transformer 17, and the other input 12is also connected to the same end terminal 15 through a currenttransformer 18 and an auxiliary transformer 19. The section 14 has anelectric power circuit breaker 20 at one end thereof for interruptingwhen an internal fault has occurred within the section.

' As is shown, the output 13 of the relay is connected through thecontacts on one side of a two-pole double throw switch or change overswitch 21 to a normally open auxiliary contact 20a of the circuitbreaker 20 through a trip coil 22 thereof.

- Further, the transistorized circuit of the relay 10 is energized froma D.C. stabilized voltage source 23 so that the same will be ready foroperation every time an accidental fault may occur in the section of thetransmission lines. The D.C. source 23 is also connected to a stabilizedA.C. auxiliarly voltage source (not shown), that is energized from andsynchronized with the source of the transmission lines.

It should be clearly understood that such a relay and circuit breaker asdescribed above are also provided on the other end terminal of thesection 14 (not shown), whereby a fault, which may bea short circuitoccurring between or among two or three phase lines of the transmissionlines, is effectively removed from the section by the cooperated openingof both of the breakers which will be tripped due to the output signalsof both of the relays located atboth ends of the transmission lines,respectively.

To inspect the relay 10 when it is being energized from the transmissionlines, there is provided a modulating device generally shown withreference numeral 24. As shown in FIG. 2, the modulating device includesa pair of ring-modulators 25 and 26. One of the ring-modulators 25 iscomposed of a unimultivibrator 27, a single phase full wave dioderectifier 28 which is reversely biased by the output square waves of theuni-multivibrator and a transformer 29 having a primary winding 30serially connected to the A.C. inputs of the rectifier and a secondarywinding 31 with a center tap 32. Similarly, the other ring-modulator 26comprises a uni-multivibrator 33, a single phase full wave dioderectifier 34 which is reversely biased due to the output square waves ofthe unimultivibrator and a transformer 35 having a primary winding 36serially connected 'to the A.C inputs of the rectifier 34 and a seconda-The series combination of the primary winding 30 and rectifier 2 8 andthe similar combination of the primary winding 36 and rectifier 34 areconnected in parallel with each other at their input terminals as shown,and then they are connected to the stabilized A.C. source (not shown)which is synchronized with the source of the transmission lines and hasa frequency, for example of 60 Hz.

To commonly drive the multivibrators 27 and 33, there is provided anoscillator 39 which may generate a signal of frequency such as 600 Hz,which is ten times larger than the corrunercial frequency, such as, 60Hz. The frequency of 600 Hz can be established by controlling theoscillator 39 with the commercial frequency through the conductors 40.Accordingly, in some cases, a frequency multiplier may be used foroscillator 39.

The output of the oscillator 39 is fed to the inputs of mu]- tivibrators27 and 33 through diodes 41 and 42 which are connected with opposedconductive polarities to each other. Thus, the input signal to themultivibrator 27 include only the positive half waves in all of theoutput signals from the oscillator, and on the other hand, the inputsignals to the multivibrator 33 will include only the negative halfwaves in the outputs of the oscillator. It can be easily understood thatthere is a phase difference, which extends over in electric angle,between the prescribed successive positive and negative half waves.

Thus, it can be assumed that the uni-multivibrator 27 generates anuppergoing pulse train 43 at the output thereof due to the positiveinput pulses, and on the other hand, the multivibrator 33 generates anundergoing pulse train 44 at the output thereof due to the negativeinput pulses. The uppergoing pulses in the pulse train will function thebias the D.C. output of the rectifier 28 so that the A.C. currents fromthe source thereof can not pass through the rectifier during theexistence of the positive pulses. Similarly, the undergoing pulses inthe pulse train 44 will also bias the D.C. output of the rectifier 34 tosuppress the A.C. current from the source thereof which may be intendedto pass through the rectifier.

In this way, as clearly shown in FIG. 3A, the A.C. currents passingthrough the primary winding 30 of the transformer 29 are so'modulatedthat the hatched portions corresponding to the output pulses of themultivibrator 27 are removed from the basic A.C. currents of thecommercial frequency. Similarly, it will be clearly understood in FIG.3B that the A.C. currents flowing through the primary winding 36 of thetransformer 35 will remove the hatched portions corresponding to theoutput pulses of the multivibrator 33 from the original A.C. currents inthe commercial frequency. It will be easily understood that in thehatched portions in FIGS. 3A and 3B, the phase difference shown by thedisplacement of the pulses between the hatched portions will occurbecause of the aforesaid treatment of the output signals of theoscillator 39.

Thus, the A.C. output voltages modulated with the pulse trains areprovided at the terminals of the secondary windings 31 and 37 of thetransformers 29 and 35, respectively.

Referring again to FIG. 1, the output terminals of the ringmodulator 24are then respectively fed to primary windings 47 and 48 each having acenter tap, of insulating transformers 45 and 46. The output pulsetrains the transformers 45 and 46 are then fed to the input 12 of therelay 10, respectively, through the secondary windings 49 and 50, eachhaving a center tap, respectively, and diodes 51 to 54 having thepolarities as shown. The center tap of the secondary winding 49 isconnected to a conductor 55 through a high pass filter 56 and the centertapof the secondary winding 50 is similarly connected to anotherconductor 57 through another high pass filter 58. The conductors 55 and57 are employed to feed the current signals which emanate from thetransmission lines to the current input 12 of the relay 10.

The high pass filters 56 and 58 will function to easily pass pulsetrains having a frequency higher than the commercial one, and willprevent A.C. currents at the commercial frequency from flowing into thering-modulator.

There is also provided a low pass filter 59 at one of the terminals ofthe secondary winding of the auxiliary current trans former 19, wherebythe current signals at the commercial frequency can pass through itdirectly to the relay input, but the inspection signals for the relay,which have frequencies higher than the commercial ones, cannot passtherethrough to the transmission line source. A decrease in theamplitude of the inspection signal will not occur.

The diodes 51 to 54 function to prevent any interferences among thepulse which respectively emanate from the halves 'from the pair ofring-modulators 25 and 26 in FIG. 2 can be easily adjusted by selectingthe time constants of the uni-multivibrators 27 and 33. Thus, it can beseen that an extremely short time interval is made between successivepulses within the inspection signals as shown in FIG. 3C, andaccordingly the relay may respond to the non-presence of the pulses.However, an integrating device, for example a capacitor 60 having smallcapacitance, is connected across the relay input 12 and serves toeffectively remove the prescribed defect by smoothing the valley betweenadjacent pulses.

To successfully transmit the pulse trains emanated from theuni-multivibrators 27 and 33 to the relay input, it is preferred thatthe ion cores of the transformers 29, 35, 45 and be made of a lowreluctance material having a high permeability, which material may be apermalloy and grain orientated silicon steel sheets. Referring again toFIG. 1, to carry out the inspection of the relay, there is provided aswitch 61 which cooperates with the change over switch 21 in order toconnect the A.C. source to the ring-modulator 24 upon the inspectionthereof. In addition a digital counter 62 which functions to start withthe exciting of the ring-modulator and a meter relay 63 which gives anindication upon the occurrence of the output signals of the relay areprovided. The meter relay stops the count of the digital counter 62 uponthe occurrence of the output signals of the relay 10 to thereby indicatea proper functioning of the relay l0.'lf no output signals occur fromrelay 10, then the relay 10 will obviously be functioning improperly andthe counter 62 will not be stopped so that an excessive count indicativeof an improperly functioning relay will occur.

It is required in order to inspect the exact operation of the relay 10,that the closing the switch 61 to force the ring-modulator 24 and thechanging over of the switch 21 from the trip circuit of the circuitbreaker 20 to the digital counter 62 and the meter relay 63 occur at thesame time. There may be also connected a phase shifter 64 between theswitch 61 and the ring-modulator 24 so as to create phase differencesbetween the current signals and voltage signals which correspond toanalogue fault conditions of the transmission lines. Thus, there willnot exist any defect in the relay 10 itself, and it can generate outputsignals with or'without a predetermined time delay corresponding to thephase difference between the input signals thereto. a

For convenience of illustration, the switch 61 and the change overswitch 21 employed to inspect the relay 10 are shown as being manualones. However of course, an automatically quick reset switch like anelectronics switch device may be employed for the quick inspection forthe relay.

Further, for convenience of illustration, a ring-modulator has beendisclosed to be excited by the auxiliary A.C. power source insynchronization with the transmission power line source. This is suitedfor the directional distance relay or the distance relay wherein it isnecessary to make a comparison of the phase differences between twoelectrical quantities which may vary in response to the extent of anyfault as well as the distances from the relay installed point to thefailed point.

There are, however, relays wherein it is necessary only to measure theamplitudes of a single electrical quantity. Thus, for example there arerelays such as an over current relay, an under voltage relay, and azero-phase over voltage relay to detect the single line grounding of athree-phase transmission line.

As an example of such relays, there is shown in FIG. 4 an over currentrelay'65 for quickly detecting the occurrence of the short circuit of aload circuit.

In the inspection system for the current relay 65, the parts which aresimilar to those of FIG. 1 are shown with the same reference numeralsand the explanation thereof is omitted in order to simplify theillustration. As is clearly shown, the parts including the elements formeasuring the voltages and the phase shifter to regulate the phasediflerences between two electrical quantities are removed from thecomponents in FIG. 1. In addition, the auxiliary A.C. source which wasnecessary to synchronize the two kinds of input signals in FIG. 1 isalso eliminated, and there is provided an oscillator, for example, aRoyers inverter or an oscillator using a uni-junction transistor forgenerating relatively high frequencies with respect to the commercialones so as to be clearly distinct therefrom. The prescribed highfrequency may then be employed for driving the ring-modulator asdescribed in the embodiment of FIG. 1.

In accordance with the present invention as described above, becausethere is employed a resultant pulse train for inspecting relay systemsin which the pulse train has distinctly higher repeating frequenciesthan those of the usual input signals for the relay to be examined orinspected, and which pulse train is modulated by an A.C. voltage havingfrequencies substantially equal to those of the aforesaid usual inputsignals of the relay, and in addition which resultant pulse train isestablished by overlapping a pair of sub-pulse trains having halffrequencies of the resultant pulse train, and since the pulse occurringtimes of the sub-pulse trains are shifted by a single pulse so as tomake a neglegible short time interval between the successive pulsesrespectively, it is possible to provide inspection signals for the relaywhich are analogically equal to the real input signals for the relay,yet the same are clearly distinct from the relay input signals. It isalso possible with the present invention to provide inspection signalsso that a a misoperation of relay using a transistorized or anintegrated circuit for detecting any fault in the circuits to beprotected by the relay which will occur. This can be achieved by addinga simple integrating memory element, for example, a, capacitor having asmall capacitance, across the output terminals of the inspection signalgenerator or the input terminals of the relay to be inspected.

Further, the inspecting device of the invention is particularly suitedfor the inspection of a distance relay and a directional distance relaywherein the phase comparison of two or more A.C. quantities is required,and this is effectively achieved by employing at least onephase-shiftable inspection I signal synchronized with the real inputsignal for the relay.

The present invention is also carried out for the inspection of a relayto be employed with heavy electric machines, for example, electric powertransformers, turbogenerators, water and gas turbine generators andpower sources for betatrons including resonance transformers. Anexemplary embodiment for the A.C. power generator is schematically shownin FIG. 5.

In FIG. 5, the relay 66 has a similar construction to the one shown inFIG. 1, and detects and responds to the variation of the internalimpedance of the alternator upon the occurrence of faults thereinthrough a potential transformer 68 and a current transformer 69. Thereis also provided a low pass filter 70 connected between the terminals ofthe secondary winding of the current transformer 69 and the currentinput of the relay 66, and a ring-modulator 71 as described above isconnected between the low pass filter and the current input of therelay.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described herein.

What is claimed is:

1. In combination with an A.C. protective relay system an apparatus forinspecting the A.C. electric protective relay system during theoperation thereof, said protective relay system having at least oneswitching transistor element therein, comprising:

a first pulse train generator emitting in operating signals of a higherfrequency than that of an actual signal applied as an input of saidrelay system upon the occurrence of a fault that the relay system is todetect, a second pulse train generator emitting in operation, pulses ofthe same frequency as the first pulse train generator, a pair ofring-modulators each separately modulating the first and second pulsetrains with an AC. voltage signal l simulative of an input that would beapplied to the relay system during actual operating conditions, meansfor combining the modulated pulse trains emitted from the pair ofmodulators into a third pulse train in which each of the pulses withinone of the first or second pulse trains is located between thesuccessive pulses within the other pulse train, said first and secondpulse train generators including means for regulating the width of eachpulse emitted therefrom,

so that an extremely small time interval exists between the successivepulses within the third pulse train, and means for applying themodulated third pulse train onto the signal input of the relay withoutany leakage of the modulated pulses in the third pulse train in adirection toward a signal from a source indicative of an actual faultthat the relay system is to detect would come from.

2. The combination according to claim 1, wherein:

said first and second pulse train generators each comprise auni-multivibrator, each having time constants which are selected so thatan extremely small time interval is provided between successive pulseswithin said modulated third pulse train.

3. The combination according to claim 1, wherein: said protective relaysystem to be inspected further includes circuit means for measuring thephase differences between voltages and electric currents in an AC.electric circuit to be protected .upon occurrence of faults in a circuitto be protected, and

a common A.C. oscillator for generating output signals having bothpositive and negative half waveforms and synchronized with the A.C.voltages in the circuit to be protected, means connecting said A.C.oscillator with said first and second pulse train generators so that thepositive half waveforms of the oscillator drive one of said pulse traingenerators and the negative half waveforms of the same oscillator drivethe other pulse train generator.

4. The combination according to claim 2, wherein:

said protective relay system to be inspected further includes circuitmeans for measuring amplitude variations in an electric quantity from apredetermined amplitude, and

means comprising an A.C. oscillator for generating output signals havingboth positive and negative half waveforms and combining them with saidfirst and second pulse train generators so that the positive halfwaveforms of the output signals of the oscillator drive one of saidpulse train generators and the negative half waveforms of the sameoscillator drive the other pulse train generator.

5. The combination according to claim 1, wherein:

said protective relay system to be inspected further includes means forcompensating at an input thereof to substantially reduce the timeintervals between the successive pulses in said third pulse train tothereby prevent said switching transistor element of the relay systemfrom misoperating by improperly turning on and off.

6. The combination according to claim 1, wherein:

said protective relay system comprises means detecting variations ininternal impedances of an electric installa tion.

7. The combination according to claim 1, wherein:

low pass filter means is further included connected between anelectrical source activated in the presence of a fault in an AC.electric circuit to be protected by said protective relay system, saidsource having a commercial frequency and connected to apply a signalinput to the protective relay system, and

means connecting the output of said pair of ring-modulators between thelow pass filter and the input of the protective rela s tern tobeinsected.

y ys l

1. In combination with an A.C. protective relay system an apparatus forinspecting the A.C. electric protective relay system during theoperation thereof, said protective relay system having at least oneswitching transistor element therein, comprising: a first pulse traingenerator emitting in operating signals of a higher frequency than thatof an actual signal applied as an input of said relay system upon theoccurrence of a fault that the relay system is to detect, a second pulsetrain generator emitting in operation, pulses of the same frequency asthe first pulse train generator, a pair of ring-modulators eachseparately modulating the first and second pulse trains with an A.C.voltage signal simulative of an input that would be applied to the relaysystem during actual operating conditions, means for combining themodulated pulse trains emitted from the pair of modulators into a thirdpulse train in which each of the pulses within one of the first orsecond pulse trains is located between the successive pulses within theother pulse train, said first and second pulse train generatorsincluding means for regulating the width of each pulse emitted therefromso that an extremely small time interval exists between the successivepulses within the third pulse train, and means for applying themodulated third pulse train onto the signal input of the relay withoutany leakage of the modulated pulses in the third pulse train in adirection toward a signal from a source indicative of an actual faultthat the relay system is to detect would come from.
 2. The combinationaccording to claim 1, wherein: said first and second pulse traingenerators each comprise a uni-multivibrator, each having time constantswhich are selected so that an extremely small time interval is providedbetween successive pulses within said modulated third pulse train. 3.The combination according to claim 1, wherein: said protective relaysystem to be inspected further includes circuit means for measuring thephase differences between voltages and electric currents in an A.C.electric circuit to be protected upon occurrence of faults in a circuitto be protected, and a common A.C. oscillator for generating outputsignals having both positive and negative half waveforms andsynchronized with the A.C. voltages in the circuit to be protected,means connecting said A.C. oscillator with said first and second pulsetrain generators so that the positive half waveforms of the oscillatordrive one of said pulse train generators and the negative half waveformsof the same oscillator drive the other pulse train generator.
 4. Thecombination according to claim 2, wherein: said protective relay systemto be inspected further includes circuit means for measuring amplitudevariations in an electric quantity from a predetermined amplitude, andmeans comprising an A.C. oscillator for generating output signals havingboth positive and negative half waveforms and combining them with saidfirst and second pulse train generators so that the positive halfwaveforms of the output signals of the oscillator drive one of saidpulse train generators and the negative half waveforms of the sameoscillator drive the other pulse train generator.
 5. The combinationaccording to claim 1, wherein: said protective relay system to beinspected further includes means for compensating at an input thereof tosubstantially reduce the time intervals between the successive pulses insaid third pulse train to thereby prevent said switching transistorelement of the relay system from misoperating by improperly turning onand off.
 6. The combination according to claim 1, wherein: saidprotective relay system comprises means detecting variations in internalimpedances of an electric installation.
 7. The combination according toclaim 1, wherein: low pass filter means is further included connectedbetween an electrical source activated in the presence of a fault in anA.C. electric circuit to be protected by said protective relay system,said source having a commercial frequency and connected to apply asignal input to the protective relay system, and means connecting theoutput of said pair of ring-modulators between the low pass filter andthe input of the protective relay system to be inspected.